Study On The Electronic Structures And Optical Properties Of Multiply Doped Lithium Niobate Crystals

Lithium niobate crystal is widely used in many fields such as optics,communication due to their excellent optical and electrical properties.This simple crystal has been chosen as one of the best materials in the volume holographic storage application.With the development of the information technology and the innovation of communication technology,due to the requirement of applications,pure lithium niobate crystal and its different doping crystal,such as photorefractive doping,anti-photorefractive doping or mixed doping,become a hot spot in the field of holographic storage materials.Various dopings could improve the photorefractive and photodamage resistance of lithium niobate crystals,and achieve non-volatile storage in doped crystals.However,the researches on lithium niobate crystal as a storage medium is mostly experimental research,the mechanism research based on the theoretical calculation of electronic structure and optical properties is still lacking,especially in the simulation study of double-doped and triple-doped lithium niobate crystals.In this paper,the electronic structures and absorption spectra of pure lithium niobate single crystal and multiple groups of doped lithium niobate crystals were studied by using the first-principles calculation method based on density functional theory.The results show that non-volatile holographic storage of lithium-acid-lithium crystal can be realized by selecting appropriate photorefractive ion single-doping or co-doping.The band gap of photorefractive ion Co²⁺doped lithium niobate system is narrower than that of pure lithium niobate crystal,with the forbidden band width as 3.32 eV.Two impurity levels appear in the band gap,which are from and the the e_g,t_2g orbitalssplitting from 3d orbitals of Co ions.In the Co,Zn co-doped lithium niobate crystal,when the concentration of Zn is lower than the threshold or reaches the threshold,the forbidden band width is 2.87 eV and 2.75 eV,respectively.The Co-doped lithium niobate crystal presents absorption peaks at2.40 eV,1.58 eV,and 1.10 eV in the visible-near-infrared light band.These peaks originate from the transition of the 3d split orbit of Co.With the anti-photorefractive ion Zn²⁺,the absorptions of the peak 1.58 eV,1.10eV absorption strengthen,it can be considered that there is charge transfer between Zn²⁺and Co²⁺,which reduces the e_g orbital electron number,but the transfer does not affect the t_2g orbital electrons.The Co ion in the crystal can serve as a deep level center?2.40 eV?or a shallow level center?1.58 eV?when co-doped with different photorefractive ions.In both cases,it is helpful to achieve optimal storage,with the doping of the near-threshold concentration of Zn ion.Impurity levels are generated in the band gap of the single-doped and co-doped lithium niobate system of photorefractive ions Cu and Ru.These levels are contributed by the Cu 3d orbital or Ru 4d orbital in single doping crystals.In co-doped crystal,the deep level is contributed by the 3d orbit of Cu,and the shallow level is contributed by the 4d orbit of Ru.The Mg orbital does not contribute any level in the band gap of the co-doped system.The Cu,Ru co-doped system forms absorption peaks near 2.94 eV?422 nm?and 2.40 eV?517 nm?.The former is derived from the 3d orbital electron transition of Cu,and the absorption is enhanced with respect to Cu single doping.The latter is derived from electron transition from the 4d orbital of Ru,however the absorption is weaker than that of Ru single doped crystal.Therefore,it could be considered that these different changes of the two absorption peaks are related to the electron transfer between Cu and Ru ions.The absorption spectrum analysis shows that increasing the concentration of photorefractive ions could augment the concentration of the storage center,and then improve the dynamic range and sensitivity of the holographic storage.The doping of Mg at the threshold concentration affects only the absorption edge of the system,which helps to improve the anti-light damage performance of the crystal and to improve the non-volatile holographic storage characteristics.